Head to Howling Cave just east of Skingrad; it should be marked on the map. Proceed through the front of the cave until they reach the rear. There are only a few rats and mudcrabs present.

At the rear of the cave, guarded by two level-appropriate undead, is a false rock which hides a door that leads deeper into the cave. Inside are the five necromancers as well as an assortment of level-appropriate undead that they have to kill. The compass should point to each of the necromancers, who at high levels may have made the transition to lich.

They all are standing in different rooms but may come forward as soon as the first necromancer is attacked. Even at a high level they can be hard to kill. After killing them if chosen, loot all the dead for leveledweapons, potions and scrolls. Each necromancer carry a large amount of restore health potions.

One of Meridia's followers has told me that the Daedra can be summoned if something from the corpse of an undead creature is left as an offering at the statue. [You must be level 10 to begin this quest.]

Update: After summoning Meridia:

After I made the correct offering, Meridia appeared and spoke to me. The Daedra told me of a cult of necromancers in the Howling Cave who have been stealing from graves in order to raise an undead army. Meridia wants the necromancers killed. I can reach them from a secret door in the back of the cave.

Update: After killing all 5 necromancers:

I have killed the necromancers in Howling Cave. I should return to the Shrine of Meridia.

Update: After speaking with Meridia again:

Meridia was gratified that I had performed the task assigned to me. As a reward, I have been given the Ring of Khajiiti.